The present invention relates generally to polyolefin foams and, more particularly, to extruded foam sheets comprising a blend of polypropylene and low density polyethylene.
Polyolefin foams, particularly polyethylene foams, and methods for manufacturing such foams are well known in the art. See, e.g., U.S. Pat. Nos. 5,348,984 (Lee), 5,462,974 (Lee), and 5,667,728 (Lee), the disclosures of which are incorporated herein by reference thereto. One of the most common polyethylenes used is low density polyethylene (LDPE).
While polyethylene (PE) possesses a number of beneficial physical and chemical properties when used to produce a foamed sheet, a disadvantage of PE is that extruded foam sheets made therefrom have a flexural modulus that is lower than would otherwise be desired for certain applications. For example, PE foams are commonly used to manufacture a type of flotation article known as a xe2x80x9cbodyboard,xe2x80x9d which is a generally planar foamed structure of various shapes and sizes that allows individuals to be carried forward by oceanic waves as the waves break near the shore, i.e., to ride or xe2x80x9csurfxe2x80x9d the waves. Individuals engaging in such activity generally prefer that the bodyboard be as stiff as possible as this allows the individual to maneuver the board to control somewhat the individual""s direction of travel relative to the wave surface upon which the individual is xe2x80x9csurfing.xe2x80x9d Too much flexure in the bodyboard detracts from this ability to maneuver the board along the wave surface. Accordingly, it would be desirable to increase the stiffness of the foam used to make bodyboards.
Another shortcoming of foam made from PE is that such foam has a lower temperature resistance than would otherwise be desired in certain applications, e.g., when using PE foam as an insulation material or as a container for microwavable foods. When exposed to relatively high temperatures as experienced in such applications, PE foam tends to soften and its shape becomes distorted.
One possible solution is to produce foam from polypropylene (PP). PP foams are typically stiffer and have greater temperature resistance than PE foam. However, molten PP generally has poor melt strength, making it difficult to produce acceptable quality foam, i.e., one having a uniform array of fully-formed, closed cells. Further, PP foams are often brittle and allow cracks to propagate readily through the foam. In addition, PP foams generally exhibit poor thermoformability such that it is difficult to thermoform such foams into desired shapes.
Accordingly, a need exists in the art for an improved foam that overcomes the foregoing disadvantages.
That need is met by the present invention, which, in one aspect, provides a foam, comprising a blend of low density polyethylene and polypropylene having a melt tension of greater than about 20 centinewtons at 200xc2x0 C. The low density polyethylene is present in the blend at a weight percentage ranging from about 1 to about 30.
Another aspect of the invention pertains to a method of making a foam, comprising:
a. forming a molten blend of low density polyethylene and polypropylene, the polypropylene having a melt tension of greater than about 20 centinewtons at 200xc2x0 C. and the low density polyethylene being present in the blend at a weight percentage ranging from about 1 to about 30;
b. adding a blowing agent to the blend; and
c. causing the blowing agent to expand within the blend, thereby forming a foam.
The inventors have discovered that a blend of low density polyethylene and polypropylene having a melt tension of greater than about 20 centinewtons produces an excellent quality foam. Surprisingly, such a blend was found to exhibit extensional thickening, i.e., an increase a in extensional viscosity, when subjected to temperature and strain rate conditions encountered when polyolefins expand into a foam. As a result, the foams possess superior physical properties in comparison to foams made from PE or PP alone. In particular, the blend foams in accordance with the present invention have greater stiffness and higher temperature resistance than PE foam alone, while also being higher in quality, less brittle, and more thermoformable than foams made from PP alone.